Id have to say that Tomino's greatest work was the Gundam TV series. It's really just a masterpiece. (Although my favorite, personally, is Zeta Gundam.)

His worst? I'd say F91.

For reference, here's a list of Tomino's credits. I don't think its complete, but its all I can remember:

He started out doing storyboards for Tetsuwan Atom (!) and he also worked on Ashita No Joe and Princess Knight. His first directorial project was Triton of the Sea, and the first giant robot show he directed was Yuusha Reideen. And then came...
Zambot 3
Daitarn 3
Gundam (TV series and movies)
Ideon
Xabungle
Dunbine
L-Gaim
Z Gundam (TV series and movies)
Gundam ZZ
Char's Counterattack
Gundam F-91
V Gundam
Turn-A Gundam
Brainpowerd
Overman KingGainer

Garzey's Wing is in there somewhere too, though I don't remember where. He also wrote a whole bunch novels (mostly Gundam-related).

It's hard to top the epic Ideon: Be Invoked... although I'd say the Gundam movie trilogy probably just edges it out for his best work. Zeta Gundam's up there too, as are the TV equivalents of first Gundam and Ideon. L-Gaim's quite good too, from what I've seen of it.

Worst? Well haven't seen anything bad from him yet. Gundam F91 would definately stand out as the worst of his work that I've seen, not that its bad, it just can't hold up to the rest of his stuff. Still got a lot of Tomino stuff left to see (Zambot, Daitarn, Xabungle, Gundam ZZ, Turn A Gundam, Gainer, etc...), though, will be seeing Brain Powered soon. Gotta love rightstuf.com, got the entire 26 episode series for less than $30.

Also, it occurs to me now that Gundam ZZ might qualify as Tomino's worst...

I made sure to scoop it up during a Bandai sale a week or two ago, where they were giving something like 40% off. Even without the discount that particular series was quite cheap, as many are if you wait for the right time. Got the Gasaraki box set for $30 for example Got Aura Battler Dunbine pretty cheap as well.